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Monday, November 26, 2007

Carbon nanotubes to be replaced

Carbon nanotubes to be replaced by MoSIx nanowires in high-tech devicesCarbon nanotubes have long been touted as the wonder material of the future. Applications cited for carbon nanotubes range from super fast computers and ultra small electronics through to materials that are lightweight yet super strong and tougher than diamond. Several techniques have been devised for producing carbon nanotubes but, getting these materials and devices from the laboratory to the marketplace is obstructed by one inherent problem. Scaling up laboratory production techniques to produce commercial quantities of high quality, high purity carbon nanotubes is a difficult process. But this is set to change with another type of recently discovered nanotube currently under investigation.

This promising new material is molybdenum-sulfur-iodine nanowires. Researchers from Jožef Stefan Institute have investigated the atomic and electronic structure of molybdenum-sulfur-iodine molecular nanowires as well as their basic transport, optical and mechanical properties. The research has now been published in a special edition of the open access journal, AZoJono and can be accessed in its entirety at http://www.azonano.com/Details.asp?ArticleID=2039.

This special edition of AZoJono* features a number of papers from DESYGN-IT, the project seeking to secure Europe as the international scientific leader in the design, synthesis, growth, characterisation and applications of nanotubes, nanowires and nanotube arrays for industrial technology.

The research team of D. Dvorsek, D. Vengust, V. Nicolosi, W.J. Blau, J.C. Coleman and D. Mihailovic found that the material also known as MoSIx nanowires was relatively easy to synthesise and disperse making it highly suited to commercialisation. The properties of the nanowires point to them being suited for use in applications such as battery electrodes, tribology and field emission displays. Ongoing research will look at growth mechanisms, stoichiometry control, magnetoelasticity and electrostrictive properties.

Carbon nanotube science and technology

Carbon nanotubes are molecular-scale tubes of graphitic carbon with outstanding properties. They are among the stiffest and strongest fibres known, and have remarkable electronic properties and many other unique characteristics. For these reasons they have attracted huge academic and industrial interest, with thousands of papers on nanotubes being published every year. Commercial applications have been rather slow to develop, however, primarily because of the high production costs of the best quality nanotubes.

StructureThe bonding in carbon nanotubes is sp², with each atom joined to three neighbours, as in graphite. The tubes can therefore be considered as rolled-up graphene sheets (graphene is an individual graphite layer). There are three distinct ways in which a graphene sheet can be rolled into a tube, as shown in the diagram below.

The first two of these, known as “armchair” (top left) and “zig-zag” (middle left) have a high degree of symmetry. The terms "armchair" and "zig-zag" refer to the arrangement of hexagons around the circumference. The third class of tube, which in practice is the most common, is known as chiral, meaning that it can exist in two mirror-related forms. An example of a chiral nanotube is shown at the bottom left.

The structure of a nanotube can be specified by a vector, (n,m), which defines how the graphene sheet is rolled up. This can be understood with reference to figure on the right. To produce a nanotube with the indices (6,3), say, the sheet is rolled up so that the atom labelled (0,0) is superimposed on the one labelled (6,3). It can be seen from the figure that m = 0 for all zig-zag tubes, while n = m for all armchair tubes.

SynthesisThe arc-evaporation method, which produces the best quality nanotubes, involves passing a current of about 50 amps between two graphite electrodes in an atmosphere of helium. This causes the graphite to vaporise, some of it condensing on the walls of the reaction vessel and some of it on the cathode. It is the deposit on the cathode which contains the carbon nanotubes. Single-walled nanotubes are produced when Co and Ni or some other metal is added to the anode. It has been known since the 1950s, if not earlier, that carbon nanotubes can also be made by passing a carbon-containing gas, such as a hydrocarbon, over a catalyst. The catalyst consists of nano-sized particles of metal, usually Fe, Co or Ni. These particles catalyse the breakdown of the gaseous molecules into carbon, and a tube then begins to grow with a metal particle at the tip. It was shown in 1996 that single-walled nanotubes can also be produced catalytically. The perfection of carbon nanotubes produced in this way has generally been poorer than those made by arc-evaporation, but great improvements in the technique have been made in recent years. The big advantage of catalytic synthesis over arc-evaporation is that it can be scaled up for volume production. The third important method for making carbon nanotubes involves using a powerful laser to vaporise a metal-graphite target. This can be used to produce single-walled tubes with high yield. PropertiesThe strength of the sp² carbon-carbon bonds gives carbon nanotubes amazing mechanical properties. The stiffness of a material is measured in terms of its Young's modulus, the rate of change of stress with applied strain. The Young's modulus of the best nanotubes can be as high as 1000 GPa which is approximately 5x higher than steel. The tensile strength, or breaking strain of nanotubes can be up to 63 GPa, around 50x higher than steel. These properties, coupled with the lightness of carbon nanotubes, gives them great potential in applications such as aerospace. It has even been suggested that nanotubes could be used in the “space elevator”, an Earth-to-space cable first proposed by Arthur C. Clarke. The electronic properties of carbon nanotubes are also extraordinary. Especially notable is the fact that nanotubes can be metallic or semiconducting depending on their structure. Thus, some nanotubes have conductivities higher than that of copper, while others behave more like silicon. There is great interest in the possibility of constructing nanoscale electronic devices from nanotubes, and some progress is being made in this area. However, in order to construct a useful device we would need to arrange many thousands of nanotubes in a defined pattern, and we do not yet have the degree of control necessary to achieve this. There are several areas of technology where carbon nanotubes are already being used. These include flat-panel displays, scanning probe microscopes and sensing devices. The unique properties of carbon nanotubes will undoubtedly lead to many more applications.